Infrared Radiation and Microwave Vacuum Combined Drying Kinetics and Quality of Raspberry

Infrared radiation and microwave vacuum combined drying (IR-MVD) kinetics and quality of raspberries under different microwave powers (400, 600, 800W) and vacuum pressures (45, 65, 85kPa) were investigated. Drying processes were found to be well fitted with Henderson and Pabis model for infrared radiation drying (IRD) and Midilli model for microwave vacuum drying (MVD). Drying time was reduced and moisture effective diffusivity (D-eff) was promoted with the increase of microwave power. In terms of vacuum pressure, the most rapid moisture diffusion was noted at 65kPa. IR-MVD led to brighter products but caused decreases in redness values. The highest a* value (11.33), lowest E value (12.66) and largest h degrees value (9.01) were noted at the condition of 600W and 85kPa. No significant difference was found between crispness values for different IR-MVD conditions. Good rehydration property was obtained under the strongest microwave power and the highest vacuum pressure. Additionally, anthocyanin losses happened and antioxidant activity declined during dehydration. The highest anthocyanin retention (66.24%) was obtained at the condition of 600W and 65kPa. The maximum retention of the antioxidant capacity determined by ABTS (2,2-azinobis-(3-ethylbenzthiazoline-6-sulfonic acid)), DPPH (1,1-diphenyl-2-picrylhydrazyl) and FRAP (ferric reducing ability of plasma) were 72.12, 75.03 and 55.19%, respectively, at the vacuum pressure of 85 kPa and microwave power of 600W. By comparison, the time IR-MVD used was only 55.56% of IRD; moreover, raspberries dried by IR-MVD showed 25.63% higher rehydration ratio, 2.4 times higher crispness value, 17.55% higher anthocyanin retention and 21.21% higher DPPH radical-scavenging activity than IRD. Practical ApplicationsFresh raspberries, with juicy and soft texture, are highly perishable fruits and are frequently dehydrated so that they are available to consumers throughout the year. Dehydrated raspberries are also desirable as ingredients in dairy and bakery products. Infrared radiation and microwave vacuum combined drying (IR-MVD) was adopted for raspberry dehydration in order to achieve dehydrated products with high drying efficiency and good quality. The effects of the drying parameters, including microwave powers and vacuum pressures, on the drying characteristics and quality of raspberries were studied. The results showed that drying under high microwave power increased drying rate. High vacuum pressure promoted better product color and rehydration property as well as anthocyanin preservation and antioxidant activity. IR-MVD was proved to improve the rehydration properties, anthocyanin retention and antioxidant capacity of products in comparison with infrared radiation drying alone. Therefore, IR-MVD is a promising method to promote drying efficiency and protect the quality of raspberries. The results of this study contribute to the technological application of IR-MVD for raspberry dehydration and preservation.